With the increasing popularity of portable communication handsets, for example mobile phones, manufactures of the handsets are beginning to incorporate into the handset, in addition to the basic communication apparatus, value-added features such as calculators and video games.
A value-added feature that has been proposed for mobile phones intended for use in open areas of varying terrain, by for example ramblers and orienteers, is a barometric altimeter.
A barometric altimeter has altitude-measuring means which typically comprise a pressure sensor for measuring the atmospheric pressure and a processor for calculating the altitude. The altitude of the mobile phone is calculated using the atmospheric pressure measured by the pressure sensor in conjunction with reference pressure data stored in the altimeter, where the stored reference data corresponds to the atmospheric pressure at a known altitude.
However, as the weather changes the atmospheric pressure at a given altitude will vary. Therefore, to ensure the altimeter provides accurate altitude information it is necessary to recalibrate the altimeter whenever there is a change in the weather, which requires the altimeter to measure the atmospheric pressure when located at a known altitude.
This may not, however, always be possible when, for example, the user is in an unpolluted area and a weather change occurs, which may be exactly when the user is most likely to require accurate altitude information.
Most cellular telephone networks are or will soon be provided with means for determining the location of a mobile phone using enhanced observed time difference (E-OTD). EP A 0 303 371 describes a radio navigation and tracking system, known as “CURSOR”, which uses the spatial coherence of the signals from several radio transmitters to determine the position of a roving receiver. The signals received directly by the roving receiver are compared with those received by a fixed base station whose location is known (base) to determine their phase difference, and hence the difference in range of the base and rover from each transmitter. Three such measurements made on independent transmitters are needed for navigation and tracking in two dimensions to fix the position of the rover relative to the base station and network of transmitters. The unknown quantities calculated for each new position are the spatial x and y coordinates of the rover together with the phase offset between the local oscillators in the equipment of the two receivers.
EP A-0 303 371 also explains how the wider bandwidth signals of modulated transmissions may be used to measure the time difference between the signals received from each transmitter at the base station and the rover. In this case, the position of the peak in the cross correlation can be used as an estimator of the time difference between the two received signals, and hence the difference in distance from the transmitter of the base and rover. As with the phase-measuring system, three such measurements made on three widely-spaced transmitters suffice to calculate the spatial x and y coordinates of the rover together with the time offset between the oscillators in the two receiving stations.
E-OTD allows the longitude and latitude to be determined quite precisely. The altitude estimates are however inherently inaccurate. The reason for this inherent inaccuracy is, simply geometry. Specifically, most of the sources/receivers, i.e. base stations, are in the horizontal plane. To obtain an accurate altitude estimate, one ore more sources/receivers should be nearly overhead. Thus, location systems using E-OTD or similar techniques such as GPS are substantially less accurate in the vertical position (altitude) than in the horizontal position (longitude and latitude).
GB 2 357 582 solves this problem by a mobile telephone comprising a pressure sensor for measuring atmospheric pressure, a receiver for receiving a RF signal incorporating reference altitude and associated atmospheric pressure data and processing means for determining from the received atmospheric pressure data and measured atmospheric pressure the difference in altitude between the handset and the reference altitude. This has the advantage of allowing the mobile phone to measure the altitude without the need to recalibrate the altitude measuring means whenever a change in weather occurs. However, it is necessary to collect reference pressure at a great plurality of locations in order to compensate for local differences in air pressure. For example each base station of the cellular network could be provided with a pressure sensor and equipment for transmitting a RF signal incorporating reference altitude and associated atmospheric pressure. Thus, a considerable investment in the infrastructure has to be made before this known system can operate. Further, the maintenance of the great plurality of pressure sensors will further increase the costs of operating the system.
It is desirable to improve this situation.